Characterizing and Classifying Prokaryotes: Structured Study Notes

Characterizing and Classifying Prokaryotes

General Characteristics of Prokaryotic Organisms

Prokaryotes are the most diverse group of cellular microbes, thriving in a wide range of habitats. Only a few prokaryotes are capable of colonizing humans and causing disease. They exist in various shapes and arrangements, and possess unique reproductive and survival strategies.

• Morphology: Prokaryotic cells display a variety of shapes, including cocci (spherical), bacilli (rod-shaped), spirals, and pleomorphic forms.

• Endospores: Produced by Gram-positive genera Bacillus and Clostridium. Each vegetative cell forms one endospore, which can germinate into a new vegetative cell. Endospores are a defensive strategy against unfavorable conditions and are a concern in food safety and healthcare.

• Reproduction: All prokaryotes reproduce asexually. Main methods include:

  • Binary fission: Most common method; cell divides into two identical daughter cells.

  • Snapping division: Variation of binary fission where the cell wall breaks apart.

  • Budding: New cell grows from the surface of the parent cell.

  • Viviparity: Unique to Epulopiscium and relatives; live offspring emerge from the body of the dead mother cell.

• Cell Arrangements: Determined by the planes of division and whether daughter cells remain attached. Common arrangements include chains, clusters, and pairs.

Modern Prokaryotic Classification

Classification is based on genetic relatedness, particularly rRNA sequences. The three domains of life are:

• Archaea

• Bacteria

• Eukarya

Survey of Archaea

General Features

Archaea are prokaryotes distinct from bacteria, with unique cell wall and membrane characteristics.

• Lack true peptidoglycan in cell walls.

• Cell membrane lipids have branched hydrocarbon chains.

• AUG codon codes for methionine (like eukaryotes).

• Two main phyla: Crenarchaeota and Euryarchaeota.

• Reproduce by binary fission, budding, or fragmentation.

• Shapes include cocci, bacilli, spirals, and pleomorphic forms.

• Not known to cause disease in humans.

Extremophiles

Archaea often inhabit extreme environments.

• Thermophiles: Require temperatures above 45°C; hyperthermophiles need over 80°C. Examples: Thermococcus, Pyrodictium.

• Halophiles: Require high salinity (>9% NaCl). Many produce red/orange pigments for sunlight protection. Example: Halobacterium salinarium.

Methanogens

Methanogens are the largest group of archaea, producing methane from carbon dioxide, hydrogen, and organic acids. They play a major role in environmental methane production and are found in sediments and animal colons.

Survey of Bacteria

Deeply Branching and Phototrophic Bacteria

These bacteria are believed to resemble the earliest forms of life and are autotrophic, living in habitats similar to early Earth.

• Deeply Branching Bacteria: Examples include Aquifex (earliest branch) and Deinococcus (outer membrane like Gram-negatives, but stains Gram-positive).

• Phototrophic Bacteria: Contain photosynthetic lamellae; most are autotrophic. Divided into five groups based on pigments and electron sources:

  • Blue-green bacteria (cyanobacteria)

  • Green sulfur bacteria

  • Green nonsulfur bacteria

  • Purple sulfur bacteria

  • Purple nonsulfur bacteria

• Cyanobacteria: Gram-negative phototrophs; transformed Earth's atmosphere by producing oxygen. Some species fix nitrogen.

Table: Characteristics of Major Groups of Phototrophic Bacteria

Group	Pigment	Electron Source	Example
Cyanobacteria	Chlorophyll a	Water	Anabaena
Green sulfur bacteria	Bacteriochlorophyll	Sulfur compounds	Chlorobium
Green nonsulfur bacteria	Bacteriochlorophyll	Organic compounds	Chloroflexus
Purple sulfur bacteria	Bacteriochlorophyll	Sulfur compounds	Chromatium
Purple nonsulfur bacteria	Bacteriochlorophyll	Organic compounds	Rhodospirillum

Low G + C Gram-Positive Bacteria

These bacteria have a low guanine-cytosine content in their DNA and include several medically and industrially important genera.

• Clostridia: Rod-shaped, obligate anaerobes, many form endospores. Important pathogens: Clostridium botulinum (botulism), C. perfringens (gas gangrene), C. tetani (tetanus), C. difficile (severe diarrhea).

• Mycoplasmas: Smallest free-living cells, lack cell walls, facultative or obligate anaerobes. Colonize mucous membranes; cause "walking pneumonia."

• Bacillus: Common in soil, form endospores. Bacillus thuringiensis produces insecticidal toxin; B. anthracis causes anthrax.

• Listeria: Contaminates food, can reproduce under refrigeration, crosses placenta (Listeria monocytogenes).

• Lactobacillus: Rarely causes disease; used in food production and microbial antagonism.

• Streptococcus and Enterococcus: Cause various diseases; some strains are multi-drug resistant. Diseases include pharyngitis, scarlet fever, impetigo, meningitis, wound infections.

• Staphylococcus: Common human inhabitant; produces toxins and enzymes. Causes bacteremia, pneumonia, wounds, food poisoning.

Table: Characteristics of Selected Gram-Positive Bacteria

Genus	Shape	Key Features	Diseases
Clostridium	Rod	Endospore-forming, anaerobic	Botulism, tetanus, gas gangrene
Mycoplasma	Pleomorphic	No cell wall, smallest cells	Pneumonia
Bacillus	Rod	Endospore-forming	Anthrax
Listeria	Rod	Reproduces at low temp	Foodborne illness
Streptococcus	Coccus	Chains, pathogenic	Pharyngitis, meningitis
Staphylococcus	Coccus	Clusters, pathogenic	Bacteremia, pneumonia

High G + C Gram-Positive Bacteria

These bacteria have a high guanine-cytosine content in their DNA and include important genera.

• Corynebacterium: Pleomorphic aerobes/facultative anaerobes; produce metachromatic granules. C. diphtheriae causes diphtheria.

• Mycobacterium: Aerobic rods, slow-growing due to mycolic acid in cell walls. M. tuberculosis (tuberculosis), M. leprae (leprosy).

• Actinomycetes: Form branching filaments resembling fungi. Genera include Actinomyces, Nocardia, Streptomyces (important in immunocompromised patients).

Gram-Negative Proteobacteria

Proteobacteria are the largest and most diverse group of Gram-negative bacteria, divided into six classes.

• Alphaproteobacteria:

  • Nitrogen fixers: Azospirillum, Rhizobium (important in agriculture).

  • Nitrifying bacteria: Nitrobacter (environmental importance).

  • Pathogens: Rickettsia (arthropod-borne diseases), Brucella (brucellosis).

  • Other: Acetobacter, Gluconobacter (vinegar production), Caulobacter (nutrient-poor water).

• Betaproteobacteria:

  • Pathogens: Neisseria (N. meningitidis, N. gonorrhoeae), Bordetella (pertussis), Burkholderia (cystic fibrosis patients).

  • Other: Thiobacillus (sulfur recycling), Zoogloea (sewage treatment), Sphaerotilus (impedes waste flow).

• Gammaproteobacteria:

  • Purple sulfur bacteria: Found in aquatic environments.

  • Intracellular pathogens: Legionella (Legionnaires’ disease), Coxiella (Q fever).

  • Methane oxidizers: Use methane as carbon/energy source.

  • Glycolytic facultative anaerobes: Catabolize carbohydrates via glycolysis and pentose phosphate pathway.

  • Pseudomonads: Break down organic compounds; Pseudomonas causes urinary tract, ear, and lung infections.

• Deltaproteobacteria: Diverse metabolism; includes Desulfovibrio, Bdellovibrio (predator of other Gram-negative bacteria), and myxobacteria (complex life cycle).

• Epsilonproteobacteria: Campylobacter (blood poisoning, intestinal inflammation), Helicobacter pylori (ulcers).

• Zetaproteobacteria: Discovered via DNA sequencing; Mariprofundus ferrooxydans is the only named species.

Table: Representative Glycolytic Facultative Anaerobes of Gammaproteobacteria

Family	Genus	Diseases/Features
Enterobacteriaceae	Escherichia, Salmonella, Shigella	GI infections, food poisoning
Vibrionaceae	Vibrio	Cholera
Pasteurellaceae	Haemophilus	Respiratory infections

Other Gram-Negative Bacteria

• Chlamydias: Intracellular pathogens; some are smaller than viruses. Most common sexually transmitted bacteria in the U.S.

• Spirochetes: Motile, corkscrew motion; diverse metabolism. Treponema pallidum (syphilis), Borrelia burgdorferi (Lyme disease).

• Bacteroids: Bacteroides inhabit digestive tracts; some cause infections. Cytophaga are aquatic, gliding bacteria important in sewage degradation.

Additional info: Academic context was added to clarify the characteristics, examples, and significance of each group. Tables were inferred and reconstructed based on typical textbook content.